Double-function sewing machines including a lock stitching mechanism and an overlock stitching mechanism have been known in the art.
In the mass production of sewn clothes, the lock stitches and the overlock stitches are most used. The lock stitching device and the overlock stitching device for producing stitches are normally mounted on a single machine frame and are driven by a common driving member. These devices have, respectively, their own needles, feeding teeth, needle plates and thread taking levers.
However, the conventional double-function sewing machines have not been provided with any arrangement for preventing switching between the lock stitching function and the overlock stitching function when the sewing machine is in operation.